Video: Polygenic Traits: Definition & Examples

If variety is the spice of life, polygenic inheritance is one genetic mechanism giving us a continuous range of possibilities. In this lesson, learn how a polygenic trait, like height, can be controlled by multiple genes.2014-02-21

Kristin has taught college Biology courses and has her doctorate in Biology.

If variety is the spice of life, polygenic inheritance is one genetic mechanism giving us a continuous range of possibilities. In this lesson, learn how a polygenic trait, like height, can be controlled by multiple genes.

Nilsson-Ehle Wheat Experiment

Your dad is six feet tall. Your mom is five feet tall. You and your siblings are all over the height chart. You are five foot two. Your sister is five feet. Your brother is five foot nine. Does this make any sense at all? Your genes came equally from your mom and your dad, so shouldn't you all just be five and a half feet, or just all six feet if the gene for height is dominant? What exactly is going on here?

I can explain. And I will. But I want to first talk about a geneticist who thought a similar question, only agriculturally. You may still be interested, because Nilsson-Ehle studied quantitative traits using wheat, and wheat is used to make a lot of delicious breads and muffins. This Swedish scientist mated pure-breeding wheat plants with red kernels with pure-breeding plants with white kernels. He knew that the red colored kernels were the dominant phenotype to white. Therefore, he fully expected to get offspring in his F1 generation with all red kernels when he crossed these two plants.

He did get a lot of red kernels, but not exactly in the way he expected. See, not all the plants had dark red kernels. In fact, most had different shades of red. If this was just a matter of incomplete dominance, he would have observed all pink kernels, but that's not what he saw either. Nilsson-Ehle continued to breed his wheat plants and study their kernel colors and what he found overtime was that there was a continuous variation from red to white in his F2 generation.

He came up with a hypothesis that this color was in fact not controlled by just one gene, but that there were additional influences playing a role. What Nilsson-Ehle had landed on was that some phenotypes, or traits, like kernel color in wheat and height in your family, are actually polygenic. Polygenic describes a trait that is controlled by two or more genes.

Polygenic Traits

If you were to graph out a trait or phenotype that is polygenic, you would notice a trend. These traits have what's called continuous variation, showing a bell curve of values for the phenotype. This is because there are multiple genes that play a role in the phenotype, and each gene could have multiple alleles. This can give a range of possible phenotypes from all the variation that's coming into play here. The more genes or alleles that contribute to the trait, the larger the possible variation can be. This appears as a continuous bell curve.

There are a lot of terms we can float around when we talk about polygenic traits because there are some overlapping concepts. For example, polygenic traits can also be quantitative because they have a continuous variation over a range of measurement. Polygenic traits can also be called multifactorial, which applies when there are multiple factors that play a role in a trait.

However, it's important for me to note something about multifactorial traits. While polygenic traits can be multifactorial, not all multifactorial traits are polygenic. In addition to multiple genes affecting the range of a phenotype, the environment can also be a factor that affects the variation of a trait. That's another topic for another lesson, but it's something to keep in mind.

Polygenic Inheritance in Humans

So, let's get back to why this matters for height in humans. Height is an example of a human trait that is affected by multiple genes. Each of these genes can have alleles, otherwise known as multiple variations of that gene. So if we plotted human height on a bar graph, we'd see this continuous variation in height for this quantitative trait. This explains why your siblings and you are all different heights - all within this range of the height of your mom and the height of your dad.

There are multiple genes controlling this height and your parents had different alleles for each gene. Another common example of polygenic inheritance in humans is in skin color. There is a wide range of skin colors that humans can have. Usually, a child's skin color is somewhere in between the skin color of either parent. However, within that range, there are continuous possibilities for what skin color a child can have.

Lesson Summary

In summary, traits that are polygenic are controlled by two or more genes. The Swedish geneticist Nilsson-Ehle used wheat and kernel color to first explore the concept of polygenic inheritance. In humans, polygenic inheritance is classically exemplified by height and skin color, two traits which we know have a wide range of possibilities. If you were to map out the different phenotypic classes that could result from a trait like human height, you would get a bar graph that resembled a bell curve.

This type of polygenic phenotypic trait is quantitative because it has a continuous variation over a range of measurement. It is also multifactorial because there are multiple factors that play a role in a trait. In the case of polygenic inheritance, these factors are multiple genes. However, it's possible for the environment to also play a role in multifactorial traits.

Summary:

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